The present invention relates to an optical direct amplifier and the control method thereof, and more specifically to an optical direct amplifier for amplifying a wavelength multiplexed signal light and a method for controlling its output level.
In the conventional optical wavelength multiplexed transmission system, an optical fiber transmission line and an optical direct amplifier are alternately connected and mounted between an optical transmission unit for transmitting a wavelength multiplexed signal light and an optical reception unit. The optical direct amplifier includes a rare-earth added fiber, an optical coupler, an optical isolator, an optical splitter, an excitation laser, an excitation laser control circuit, an O/E (opto-electric) converter, etc. One end of the rare-earth added fiber is connected to an input unit of the optical direct amplifier, and the other is connected to the optical coupler. An optical isolator 3 is connected to the optical coupler, and the optical splitter is connected to the output unit of the optical isolator. One output unit of the optical splitter is the output unit of the optical direct amplifier, and the other output unit is connected to the O/E converter. The excitation laser control circuit is connected to the O/E converter, and the excitation laser is connected to the excitation laser control circuit. As described in Japanese Patent Application Laid-Open No. 6-338874, etc., the excitation laser control circuit controls the output of the excitation laser such that the output level of the optical direct amplifier can be constant based on the signal converted into an electric signal by the O/E converter.
When the optical transmission unit transmits a wavelength multiplexed signal light having, for example, four wavelengths (hereinafter referred to as 4 channels) in the conventional optical wavelength multiplexed transmission system, the output level is controlled to be constant (for example, +8 dBm/4 channels based on +2 dBm/1 channel). However, even if two channels of the four channels are interrupted for any reason, the excitation laser control circuit controls the output level of the optical direct amplifier to be totally +8 dBm. Therefore, the optical output level per channel is controlled to be +5 dBm, which is higher than in a normal operation (4 channels). At this time, the transmission is disabled by the nonlinear effect of the optical fiber transmission line. On the other hand, when a signal light is increased in number, the optical output level per channel is lower than in the normal operation (4 channels).
The present invention aims at optically controlling the output level of an optical direct amplifier even when the number of signal lights is decreased by a fault, etc. or when the number of the signal lights is increased on the transmitting side.
The optical direct amplifier of the present invention includes a detector for detecting the number of modulated frequencies specific to an output signal light; and a control circuit for controlling the output of the optical direct amplifier depending on the number of modulated frequencies detected by the detector. The detector includes an opto-electric converter for converting an output wavelength multiplexed signal light into an electric signal; a frequency detector for detecting a modulated frequency specific to each signal light from the electric signals converted by the opto-electric converter; and a counter for counting the number of modulated frequencies. When the number of modulated frequencies specific to the signal light detected by the detector has changed, the control circuit controls the optical output level of one signal light such that the optical output level can be the same as that before the number has changed. The method of controlling the optical direct amplifier according to the present invention includes the steps of counting the number of modulated frequencies specific to the wavelength multiplexed signal light, and controlling, if the number has changed, the optical output level per signal light to be the same as the optical output level per signal light obtained before the number has changed. The optical wave length multiplexed transmission system includes an optical transmission unit, an optical reception unit, optical fiber transmission lines, and an optical direct amplifier provided between the optical fiber transmission lines. The optical direct amplifier includes a detector for detecting the number of modulated frequencies specific to an output signal light; and a control circuit for controlling the output of the optical direct amplifier depending on the number of modulated frequencies detected by the detector.
The above described optical direct amplifier maintains an initialization value as the output level per channel even if the number of signal lights increases or decreases.